It has been reported (U.S. Pat. No. 7,537,741) that gold/silver-bearing ores, after being deprived of most of their base metals such as Cu, Zn, Fe and chalcogens such as sulfur and tellurium, can be treated very efficiently with halogens for precious metals recovery. The ore is slurried in a brine of sodium chloride incorporating a minor portion of sodium bromide, and elemental chlorine is then admitted to the system at near ambient temperature (40-50° C.). The high oxidation potential of the chlorine leads to the formation of some bromine from the bromide in the slurry, and the mixed halogens (chlorine, bromine) lead to a fast (a few hours) and rather complete precious metals recovery. This process is a closed loop approach, which means that the brine, after separation from the barren solution and precious metal recovery, is used as a source of elemental chlorine. This production of chlorine is done by the electrolysis of the brine in a standard electrolytic cell with either a membrane or a diaphragm for the separation of the cathodic compartment from the anodic one.
This method is submitted to significant limitations when implemented. First, the brine collects many types of ions in the course of the gold extraction, particularly elements of the alkaline earth group, such as calcium and magnesium. As state of the art materials used as diaphragms or membranes are highly sensitive to these contaminants, extensive purification of the brine is thus required prior to electrolysis. Secondly, the solubility of chlorine in water is rather low, 0.091 mole/l at 25° C. (Advanced Inorganic Chemistry, A. Cotton and G. Wilkinson, Interscience Publishers, 1972, p. 476). This solubility is further decreased if a brine of NaCl rather than pure water is used. Chlorine addition requires periods of time counted in hours rather than minutes. In turn, finally, this long addition time leads to secondary reactions of chlorine and bromine with remaining base metals in the ore, mainly iron, and with sulfur as sulphide, thus increasing the overall consumption of halogens.
There thus remains a need for an improved method and system for precious metals extraction by the halogens allowing recycling halogens.